The invention relates in general to the operation of a receiver in order to find the electric signal to be received, and in particular to the utilisation of a control signal provided in a predetermined rhythmic pattern in a situation where the receiver must find the signal among several possible frequencies and synchronise the reception according to the signal properties.
In such radio communications systems that have several data transmission frequencies and variable uses with respect to area and/or time, the receiver must, prior to beginning the reception proper, find the desired signal and synchronise its operation in order to interpret the content of the signal. Finding the signal means that the receiver is tuned to exactly that frequency where the signal is located. In the synchronisation process the receiver must find out where each separate symbol pertaining to the signal begins, and at what rate the signals succeed to each other.
The present application pays special attention to the I-CO Global Communications satellite telephone system, which is based on ten communications satellites with a so-called medium-high orbit (roughly 10,000 km). The satellites orbit the earth at regular intervals on two mutually perpendicular orbits with an inclination of 45.degree.. Each satellite comprises an antenna arrangement with a power pattern of 121 narrow radiation lobes, which together cover the coverage area of said satellite on earth. The coverage area means the whole area from which the satellite is seen more than 10 degrees above the horizon. The operational frequency range of the system is roughly 2 GHz, and it utilises TDMA, Time Division Multiple Access.
As a concept, the system defines a so-called CCS carrier (Common Channel Signalling), which means a given carrier frequency reserved for signal acquisition, synchronisation and distribution of general communications information. Globally there are reserved 120 frequencies for CCS carriers, and these frequencies are further grouped into regional and local frequencies. When a certain satellite moves on its orbit, its coverage area moves along the surface of the earth. The satellite changes the transmitted CCS frequencies in between the separate radiation lobes, so that in a given geographic area, there are always received the same frequencies. A receiver located on earth or near the surface of the earth stores the eight location-connected local CCS frequencies to a non-volatile memory; consequently, when it is switched off and back on, it searches a signal from among said eight frequencies. If a signal is not found, the receiver next studies the 40 regional frequencies, and if there still is no signal, finally all 120 global frequencies.
According to FIG. 1, a transmission with each CCS frequency consists of several multiframes 10, which are divided into 25 slots 11. Each slot includes 120 symbols 12. According to current definitions, the symbol rate in the system is 18,000 symbols per second, but it may be increased to 36,000 symbols per second in the future. The first slot in the frame comprises a BCCH (Broadcast Control Channel) burst 13, which is BPSK (Binary Phase Shift Keying) modulated and contains, among others, communications data and a 32 symbols long reference sequence 14, which is important for synchronisation. The location and form of the reference sequence inside the BCCH burst will be essentially fixed and known. Two successive slots contain a FCH (Frequency Channel) burst 15, which is transmitted with a somewhat lower frequency than the BCCH burst and consists of pure sinus wave at the frequency of said CCS carrier; the purpose of said FCH burst 15 is to aid the synchronisation of the receiver. Other slots in the CCS carrier are empty.
For successful reception, the receiver must, after being switched on, first find the desired signal. General criteria for the signal to be found is that the timing error in the reception is .+-.1/2 symbols at the most, and that the frequency error is no more than a few percentages of the symbol rate. The nearer to zero these two error factors are, the smaller the probability that bit errors happen in the reception, and the less the reception is sensitive to the deterioration of the SIN ratio. In the prior art, there are known several methods for treating, i.e. scanning, a given number of frequencies in order to detect at what frequency or what frequencies transmission exists. As for the satellite telephone system discussed here, it has been generally suggested that the receiver observes the power profile in the frame scale with the found CCS frequency and assumes that the peak of the power profile corresponds to a BCCH burst. Thereafter the receiver picks a discrete sample series of the FCH burst and calculates therefrom a 128-sample long fast Fourier transform (FFT), the results of which are used in the coarse correction of frequency error. Detailed algorithms for carrying out frame synchronisation and eliminating frequency errors have not been given by the time of filing this application.